Interpretive Summary: Dairy calf weaning is associated with rapid increases in circulating ketone concentrations that exceed adult rates of utilization. This represents a source of energy loss that may be mitigated by ionophore supplementation. Effects of weaning and ionophore supplementation on nutrient metabolism and growth were examined in pre- and post-weaning Jersey calves. Feeding calf starter supplemented with ionophore did not have an advantageous effect on post-weaning growth, but appeared to extend the transition period from a monogastric to ruminant metabolism. Fostering a smoother digestive adaptation during weaning will reduce energy loss during this critical developmental stage increasing post-weaning health and productivity.

Technical Abstract:
Dairy calf weaning results in blood ketone concentrations in excess of mature rates of utilization and can result in excretion of ketones in urine representing a loss of energy. Lasalocid is frequently supplemented as an anticoccidial in calf starters, however in mature ruminants it is known to alter molar ratios of ruminal VFA. Effects of weaning transition and post-weaning ionophore supplementation on weight (BW), dry matter intake (DMI), gain (ADG), and blood concentrations of glucose, acetoacetate, '-hydroxybutyrate (BHBA), lactate, pyruvate, non-esterified fatty acids (NEFA), volatile fatty acids (VFA), insulin, and glucagon were examined using Jersey bull calves (n = 24) over 16 wk. Calves were blocked into groups of two according to birth date and weight and randomly assigned to receive either a commercial pelleted starter (CON), or the same diet containing lasalocid (TRT; 83 mg/kg DM). Calves were fed milk replacer from d 3 - d 34 (d 3 - 20 = 454 g/d at 12% solids; d 21 - 34 = 568 g/d at 15% solids), d 35 - 48 calves received both replacer (d 35 - 41 = 454 g/d; d 42 - 38 = 227 g/d) and free access to CON or TRT starter, and from d 49 - 112 received ad libitum CON or TRT. Body weight and jugular blood metabolite concentrations were measured and recorded weekly. Postweaning DMI, ADG, and feed : gain did not differ between CON and TRT calves. Glucose and NEFA concentrations did not differ between CON and TRT, but declined with age. Insulin and glucagon concentrations did not differ between CON and TRT, but glucagon concentrations increased with weaning. Total VFA significantly increased following introduction of solid feed at d 35 in both groups with an apparent one-wk lag in TRT VFA increases compared to CON. Ruminal acetate and butyrate concentrations were greater in CON calves than TRT calves during wk 7. Propionate concentrations did not differ between CON and TRT at any time following weaning. Blood BHBA concentrations were greater in CON than TRT during wk 8 and 9. Thus, consumption of starter supplemented with lasalocid delayed peak acetate and butyrate and lowered peak BHBA concentrations. However, supplementation at concentrations currently recommended for control of coccidiosis did not appear to be sufficient to enhance growth or efficiency during the wk 7 to 16 post-weaning interval for this size sample population.